The Field of the Invention
The present invention is related to methods for reducing NO.sub.x emissions from pollution sources, such as combustion systems. More particularly, the present invention relates to the noncatalytic, selective reduction of NO.sub.x by cyanuric acid to achieve very low levels of NO.sub.x emissions.
The Background of the Invention
One of the major problems in industrialized society is the production of air pollution from numerous sources. Air pollution can take various forms. Some of the different types of air pollutants include particulate emissions such as coal ash, partially burned coal particles, and the like, sulfur compounds such as SO.sub.2 and SO.sub.3 (sometimes collectively referred to as "SO.sub.x "), ozone, carbon oxide emissions, volatile hydrocarbon emissions, and emissions of nitrogen oxides (commonly referred to collectively as "NO.sub.x "). Pollution sources include automobiles, industrial plants, small commercial establishments, such as dry cleaners and service stations, and even nature itself.
Combustion effluents and waste products from particular types of sources have proven to be major contributors to damaging air pollution when the effluents are discharged into the atmosphere. Unless these waste products are treated before their release into the atmosphere, serious smog and air pollution problems are encountered.
It will be appreciated that high concentrations of air pollutants have serious deleterious impacts on the health and general welfare of society. Air pollution is known to aggravate certain medical conditions (such as heart and lung problems) and is known to cause problems in the environment, ranging from corrosion to acid rain.
One of the most common components found in polluted air is nitrogen dioxide ("NO.sub.2 ") which is known to be toxic. Nitrogen dioxide, which is brown in color, undergoes a series of reactions, known generally as "photochemical smog formation," in the presence of sunlight and airborne hydrocarbons. These reactions result in a marked decline in overall air quality.
While NO.sub.2 is produced from a wide variety of pollution sources, its primary source is from nitric oxide ("NO") released into the air. NO is commonly formed during combustion processes, including internal combustion engines in automobiles, hydrocarbon fuel power plants, process furnaces, incinerators, coal fired utility boilers, glass furnaces, cement kilns, oil field steam generators, gas turbines, and other similar installations.
There are two primary mechanisms for the formation of nitrogen oxides in the combustion processes. Within the high temperature portions of a flame, atmospheric oxygen can react with molecular nitrogen ("N.sub.2 ") to form NO by the high temperature "thermal fixation" mechanism.
In addition, fuels which contain large amounts of nitrogen chemically bound within the fuel structure may produce significant NO.sub.x emissions as a result of the oxidation of the fuel nitrogen during the burning process. This source of NO.sub.x emission (often termed "fuel NO.sub.x ") is the predominant source of NO.sub.x with the combustion of coal, heavy oils, biological and agricultural residues, and some municipal, industrial, and agricultural wastes.
Since NO is the primary oxide of nitrogen which is stable at the high temperatures encountered in these types of combustion processes, NO is the predominant nitrogen emission product. At normal atmospheric temperatures, however, the equilibrium between NO and NO.sub.2 favors NO.sub.2. Hence, NO formed by combustion is generally discharged into the atmosphere as NO, and only subsequently converted to NO.sub.2. In order to control NO.sub.2 emissions, therefore, it is necessary to eliminate NO before it enters the atmosphere.
There have been considerable efforts in the art to find effective ways to remove oxides of nitrogen from waste gases so that these waste gases may be discharged to the atmosphere without harm to the environment.
Because the "thermal fixation" of atmospheric nitrogen is exclusively a high temperature phenomenon, occurring above 2800.degree. F., it has been possible to achieve significant reductions in NO.sub.x emissions from the combustion of nitrogen-free fuels (such as natural gas or gasoline) by reducing the overall temperature in the combustion zone. This is accomplished using techniques such as exhaust gas recirculation in automobiles or flue gas recirculation in utility boilers.
Fuel NO.sub.x formation is most easily controlled by limiting the amount of oxygen present during the period in which the nitrogen species are being evolved from the fuel matrix. Techniques such as a staged combustion, overfire air addition, and "burners out of service" will use this concept to limit fuel and nitrogen oxidation.
More recently, it has been recognized that limited amounts of hydrocarbon fuels, particularly those which do not contain fuel nitrogen, can be used to effectively incinerate NO formed in the main combustion zone by creating a fuel rich (oxygen deficient) environment downstream of the primary combustion zone. This technique is generically termed "reburning," and like the other combustion modification techniques, is capable of producing overall NO.sub.x reductions in excess of 50% under optimized conditions.
Unfortunately, at the present time, none of the combustion modification techniques are capable of producing very high levels of NO.sub.x control in the range of approximately 80% to 90%. To achieve extremely low NO.sub.x emission levels, it is necessary to utilize some type of downstream, effluent gas cleanup system.
It has been found in the art that removal of NO.sub.2 from a combustion effluent stream is relatively easy since it reacts with water and air to form nitric acid. NO.sub.2, therefore, is commonly removed by aqueous scrubbing. If a base, such as ammonia, is added to the scrub water, the nitrogen scrubbing process is facilitated and ammonium nitrate is produced. If limited amounts of NO are present along with the NO.sub.2, the NO may be coscrubbed, thereby yielding ammonium nitrate.
Most chemical scrubbing techniques are subject to the limitation that they are only effective for mixtures of nitrogen oxides which are predominantly NO.sub.2, rather than predominantly NO. This presents a problem because NO is the predominant species at the high temperatures generally encountered in flue gases. As a result, various processes have been developed in the art for oxidizing NO to NO.sub.2 so that the relatively inexpensive and convenient scrubbing processes may take place.
Several processes known in the prior art involve contacting the gaseous flow which includes NO, with various organic compounds (such as aldehydes, alcohols, ketones, organic acids, and the like) in the presence of oxygen. By such processes, the NO is oxidized to NO.sub.2 which can then be removed by scrubbing as described above. None of these processes, however, are capable of efficiently producing very low levels of NO.sub.x emissions.
An alternative approach for removing NO from flue gases and other streams of pollutants is to reduce NO to nitrogen and water, which may then be discharged to the atmosphere. Reduction of NO.sub.x may be accomplished with or without catalytic assistance. Practically, the noncatalytic processes are preferable because they are not subject to the usual disadvantages of employing catalysts. Some of these additional disadvantages include higher expense associated with the catalyst, the potential of catalyst plugging, the expense and difficulty of contacting the combustion effluents with the catalyst, and the danger that the catalyst will disintegrate and be emitted into the atmosphere.
Alternatively, NO.sub.x reduction processes often teach the removal of NO.sub.x from flue gases by reduction of the NO by the addition of ammonia, urea, or ammonia precursors, alone or in combination with some other combustional material, while the waste gas is at a relatively high temperature (generally from about 700.degree. C. to about 1200.degree. C.).
An example of such an NO reduction process is described in U.S. Pat. No. 3,900,554 to Lyon, issued Aug. 19, 1975, entitled "Method for the Reduction of the Concentration of NO in Combustion Effluents Using Ammonia." The process disclosed in that patent teaches the reduction
Perry shows two experiments in which cyanuric acid is used to reduce No. In one of these experiments a laboratory procedure was used to convert a batch of cyanuric acid into HNCO which was then stored and used in subsequent experiments. These experiments are done in a stainless steel reactor and involved contacting the HNCO with a mixture of NO and argon at temperatures in the range of 582.degree. to 647.degree. C. In the second experiment cyanuric acid was directly contacted with an oxygen containing combustion effluent at temperatures in the 20.degree. to 400.degree. C. range, i.e., a flow of combustion effluents is passed through a bed of solid cyanuric acid, the amount of cyanuric acid which is added to the flowing combustion effluents being dependent on the temperature of the bed of solid acid. After passing through the bed of acid the combustion effluents flow through a second reactor packed with stainless steel balls at a temperature between 450.degree. and 900.degree. C.
Several limitations of the teachings of this reference are to be noted. The reference teaches two methods of contacting cyanuric acid with NO containing gases. In one of these methods the cyanuric acid is decomposed to HNCO which is then purified and stored prior to contacting with the NO containing gas. While this method may be quite satisfactory for laboratory experiments, in any practical application the storage of HNCO would be unacceptable since HNCO is a highly toxic material with very unfavorable storage characteristics. It would be an advancement in the art to provide a method which did not require that hazardous intermediates be stored.
The reference also shown an example in which cyanuric acid is contacted directly with oxygen containing combustion effluents. In this example, however, the reduction of NO was achieved only by using a great excess of cyanuric acid over NO. Use of lessor amounts of cyanuric acid actually caused the NO level to increase, i.e., adding cyanuric acid causes the NO level to increase from an initial value of 600 ppm to higher and higher levels finally climbing to 8900 ppm. Only after enough cyanuric acid has been contacted into the combustion effluents to produce this high level of NO does further addition of cyanuric acid cause a decrease in the NO level.
An additional limitation of this reference relates to the fact that both sets of experiments were done in stainless steel reactors. It si generally believed that stainless steel is a catalyst for the decomposition of HNCO, for the reduction of NO by CO and for other reactions. It would be an advancement in the art to provide a process which reduced NO in combustion effluents but which did not require the presence of stainless steel or other catalytic materials.
As a result, while the use of cyanuric acid provided some hope for obtaining low levels of NO.sub.x emission at moderate temperatures (600.degree. C. to 1100.degree. C.), in actual combustion application, the reduction of NO.sub.x by cyanuric acid has been found to be ineffective at moderate temperatures, and at best, no more effective than existing techniques at high temperatures.
Another group of pollutants which are of major importance are the sulfur oxides (generally collectively designated "SO.sub.x "). Sulfur oxides are primarily emitted in the form of sulfur dioxide ("SO.sub.2 "), with small amounts of accompanying sulfur trioxide ("SO.sub.3 "). Since there is no harmless gas phase sulfur species analogous to N.sub.2, combustion modification has not been useful for controlling SO.sub.x emissions. Exhaust gas cleanup systems, however, including both wet scrubbing and spray drying techniques, are well known and effective.
High temperature (1800.degree. F. to 2800.degree. F.) injection of dry, pulverized limestone has also been used to reduce sulfur emissions. In addition, several recent investigations have shown that CaO injection produces very poor SO.sub.x reduction, but hydrated lime (Ca(OH).sub.2) is effective in reducing SO.sub.x emissions. None of the current literature, however, shows that dry sorbent injection can be directly combined with reducing agent injection to achieve optimum NO.sub.x and SO.sub.x control simultaneously and with relatively small capital cost. Such a process would be a major advancement in the art.
From the discussion above, it is apparent that what is currently needed in the art are methods for the selective, noncatalytic reduction of NO.sub.x which produce NO.sub.x emissions well below those obtainable using prior art methods. It would be an advancement in the art to provide such methods which employed a cyanuric acid process which was effective in actual practice. It would be a further advancement in the art to provide methods for simultaneously controlling NO.sub.x and SO.sub.x emissions.
Such methods are disclosed and claimed below.